•Polypyrrole coatings were successfully deposited on rheo-cast Al- 4.5% Si alloy.•Eutectic silicon and intermetallic particles increase the electrodeposition rate.•Polypyrrole coatings can relatively protect the Al-Si alloy in 0.6 M NaCl solution.•Barrier effect and passivation protection mechanism can be suggested.•Casting related defects on the coating’s surface affect the corrosion protection. Electropolymerization of polypyrrole coatings in the presence and absence of sodium nitrate was applied on rheo-cast Al-4.5% Si alloy and pure aluminum. The results showed that the eutectic silicon phase and intermetallic particles in the alloy’s microstructure increase the electrodeposition rate in comparison to the pure aluminum substrate. The electrochemical and microstructural studies show that the polypyrrole coatings are able to protect the surface due to the barrier properties and the passivation protection provided by the reduction of the conductive polymer. The coating electrodeposited from sodium nitrate-containing electrolyte presented improved protection for longer immersion time. Localized formation of a thick oxide layer as a result of the drastic galvanic coupling at the polypyrrole/aluminum interface leads to blister formation and failure of the coating. It was revealed that the coating could be deposited into the porosities produced by the casting related defects, but in most cases, this affects the corrosion protection leading to imminent failure.